Recently, a phone call between two communication terminals is performed through a relay processing device. Specifically, a call terminal calls using a phone number of the relay processing device as the phone number of a called terminal, and thus communication between the calling terminal and the relay processing device is established. Then, the relay processing device calls the called terminal, and thus communication between the relay processing device and the called terminal is established. The relay processing device relays the phone call between the calling terminal and the called terminal (for example, see Patent Document 1).
When relaying the calling terminal and the called terminal by using the relay processing device, there is a disadvantage that the phone number of the calling terminal is not transmitted to the called terminal, and thus it cannot be aware of which calling terminal has called. To solve this problem, Patent Document 2 discloses a system that performs the following process to confirm from which calling terminal the called terminal is receiving a call. Firstly, plural phone numbers are prepared, which are used when the relay processing device establishes communication with the called terminal. Then, a user of the called terminal allocates the phone numbers of terminals that may be the calling terminals to the respective phone numbers. The relay processing device determines which phone number is used to call the called terminal according to the allocation.